According to the fetch spec, there is a CORS-preflight request in order to verify if the server understand the CORS protocol.

The web service documentation recommends providing a meaningful User-Agent string, but User-Agent is not a safelisted CORS header, so spec-adhering tools will not be able to perform CORS requests correctly. Currently, Firefox 69+ is one having issues.

Firefox issue related to updates in the fetch spec

Relevant source on MBS: https://github.com/metabrainz/musicbrainz-server/blob/master/lib/MusicBrainz/Server/Controller/Role/CORS.pm#L22